Effect of concentration on solution state redox activity- A bridge between polymer physics and electrochemistry

Redox-active polymers are popular electroactive materials for energy storage due to their unique electronic properties. In solution-state electrochemistry, polymer concentration can be varied. Polymer chains behave differently at different concentrations. We hypothesize that concentration variation from a dilute to a semi-dilute regime can change their redox activity due to change in the electron hopping mechanism. In this study, we investigate the redox activity of phthalimide-based polymers in an organic electrolyte system at different concentrations. These are non-conjugated polymers that have redox active pendant groups attached to their backbone. They gain electrons during charge and reduce electrons during discharge. The electron transfer starts with the diffusion of the redox-active species followed by molecular orientation and then the transfer of electrons to other redox-active groups (self-exchange) or transfer to the current collector (heterogenous). We consider the diffusion cooperative model, where the electron self-exchange occurs with the physical diffusion of the redox-active sites. The molecular orientation occurs due to Brownian motion. An electron hops from one redox unit to another only when they are in their closest proximity. We focus on the variation in the homogenous and heterogenous rate constants, and diffusion coefficient at each concentration. A fixed trend of variation of kinetic parameters and the factors affecting such a trend have been described.